Resin viscosity detection in additive manufacturing

ABSTRACT

A method of measuring the viscosity of a resin in a bottom-up additive manufacturing apparatus, includes the steps of: (a) providing an additive manufacturing apparatus including a build platform and a light transmissive window, said build platform and said window defining a build region there between, with said window carrying a resin; (b) advancing said build platform and said window towards one another until said build platform contacts said resin; (c) detecting the force exerted on said build platform by said resin; and (d) generating in a processor a viscosity measure of said resin from said detected force.

RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.16/845,416, filed Apr. 19, 2020, which application claims priority toU.S. Provisional Application Ser. No. 62,839,187, filed Apr. 26, 2019,the disclosure of which is hereby incorporated by reference in itsentirety.

FIELD OF THE INVENTION

The present invention concerns stereolithography methods and apparatus,particularly those for carrying out bottom-up additive manufacturing,such as continuous liquid interface production.

BACKGROUND OF THE INVENTION

A group of additive manufacturing techniques sometimes referred to as“stereolithography” creates a three-dimensional object by the sequentialpolymerization of a light polymerizable resin. Such techniques may be“bottom-up” techniques, where light is projected into the resin on thebottom of an object growing on a build platform above a lighttransmissive window, or “top down” techniques, where light is projectedonto the resin on top of the growing object, which is then immerseddownward into the pool of resin.

The introduction of more rapid stereolithography techniques sometimesreferred to as continuous liquid interface production (CLIP), coupledwith the introduction of “dual cure” resins for additive manufacturing,has expanded the usefulness of stereolithography from prototyping tomanufacturing (see, e.g., U.S. Pat. Nos. 9,211,678; 9,205,601; and U.S.Pat. No. 9,216,546 to DeSimone et al.; and also in J. Tumbleston, D.Shirvanyants, N. Ermoshkin et al., Continuous liquid interfaceproduction of 3D Objects, Science 347, 1349-1352 (2015); see alsoRolland et al., U.S. Pat. Nos. 9,676,963, 9,453,142 and 9,598,606). Forsuch purposes, bottom-up stereolithography is preferred, as the pool ofresin (sometimes provided on a “window cassette”) can be more shallowand hence smaller in volume.

The production of accurate three dimensional objects at even reasonablespeeds requires satisfactory control, made in light of variables such aslight intensity, resin temperature, resin viscosity (which can vary asresin temperature varies), object geometry, and more. Parameters such asresin viscosity can vary from resin batch to batch, can depend on theage of the resin, can depend on how the resin has been stored, and evenvary based on the current temperature of the resin. Accordingly, thereis a need for methods and apparatus for measuring resin viscosity closein time to the use of the resin for the production of objects.

SUMMARY OF THE INVENTION

According to some embodiments of the present invention, a method ofmeasuring the viscosity of a resin in a bottom-up additive manufacturingapparatus, comprising the steps of: (a) providing an additivemanufacturing apparatus including a build platform and a lighttransmissive window, said build platform and said window defining abuild region there between, with said window carrying a resin; (b)advancing said build platform and said window towards one another untilsaid build platform contacts said resin; (c) detecting the force exertedon said build platform by said resin; and (d) generating in a processora viscosity measure of said resin from said detected force.

In some embodiments, the generating is carried out with an empiricalmodel.

In some embodiments, the generating is carried out with a regressionmodel (e.g., multi-variate linear regression, random forest regression,etc.).

In some embodiments, the generating is carried out with a machinelearned model.

In some embodiments, the apparatus includes a force sensor operativelyassociated with said build platform and/or said light transmissivewindow, and the detecting step is carried out by detecting force exertedon said force sensor.

In some embodiments, the force sensor comprises a strain gauge.

In some embodiments, the window is stationary in the lateral (X, Y)dimensions.

In some embodiments, the window is permeable to oxygen.

In some embodiments, the method further includes: (a) recording saidviscosity measure in association with a resin batch identity (e.g., as aquality control measure of said resin); and (b) producing an object onsaid additive manufacturing apparatus, wherein said producing ismodified based on said viscosity measure (e.g., by speeding or slowingproduction).

In some embodiments, an apparatus useful for making a three-dimensionalobject from a polymerizable resin includes (a) a build platform on whicha three-dimensional object can be made; (b) a light transmissive windowhaving a build surface operatively associated with said build platform,said build platform and said build surface defining a build regiontherebetween, said window configured to support a resin pool thereon;(c) an elevator assembly operatively associated with said build platformand/or said window, said elevator assembly configured for advancing saidbuild platform and said window member away from one another to draw saidpolymerizable liquid into said build region; (d) a light engineoperatively associated with said window and positioned to irradiate saidbuild region with light to form a growing three-dimensional object fromsaid resin; (e) a force sensor operatively associated with said platformand/or said window and configured to detect force exerted on said buildplatform upon contacting said resin; and (f) a controller operativelyassociated with said carrier platform, said light engine, and said forcesensor, said controller configured to generate a viscosity measurementfor said resin from said detected force.

In some embodiments, the window is stationary in the lateral (X, Y)dimensions.

In some embodiments, the light engine comprises a light source (e.g., alaser) in combination with a patterning array (e.g., a liquid crystaldisplay array or a digital micromirror array).

In some embodiments, the force sensor comprises a strain gauge.

In some embodiments, the controller generates a viscosity measurementwith an empirical model.

In some embodiments, the controller generates a viscosity measurementwith a regression model (e.g., multi-variate linear regression, randomforest regression, etc.).

In some embodiments, the controller generates a viscosity measurementwith a machine learned model.

In some embodiments, the window is permeable to oxygen.

In some embodiments, the controller is configured to (a) record saidviscosity measure in association with a resin batch identity (e.g., as aquality control measure of said resin); and (b) produce an object onsaid additive manufacturing apparatus, wherein said producing ismodified based on said viscosity measure (e.g., by speeding or slowingproduction).

R. Truong, Continuous Liquid Interface Production with Force Monitoringand Feedback, PCT Application WO 2018/111533 (published 21 Jun. 2018),describes the use of a force sensor during production of an object on anadditive manufacturing apparatus to enhance efficiency of production,but does not suggest that such a force sensor could be used to measureresin viscosity prior to production of the object.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph illustrating the forces exerted on the build platformin on example of a bottom-up stereolithography apparatus after printstart. During the initial descent of the platform towards the cassette,the resin exerts compressive forces on the platform.

FIG. 2 is a schematic diagram illustrating an apparatus useful forcarrying out the present invention, prior to initiating production of anobject and prior to determining resin viscosity.

FIG. 3 is a schematic diagram similar to FIG. 2, except that the buildplatform has now been advanced down to contact the resin top surfaceportion, so that impact can be detected and resin viscosity determined.

FIG. 4 is a flowchart demonstrating the processing of build platformforce and height time series data, which is fed into a regression modelfor the prediction of viscosity.

FIG. 5 is a graph illustrating the actual viscosity (measured byviscometer) alongside the model's predicted viscosity.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The present invention is now described more fully hereinafter withreference to the accompanying drawings, in which embodiments of theinvention are shown. This invention may, however, be embodied in manydifferent forms and should not be construed as limited to theembodiments set forth herein; rather these embodiments are provided sothat this disclosure will be thorough and complete and will fully conveythe scope of the invention to those skilled in the art.

Like numbers refer to like elements throughout. In the figures, thethickness of certain lines, layers, components, elements or features maybe exaggerated for clarity. Where used, broken lines illustrate optionalfeatures or operations unless specified otherwise.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a,” “an” and “the” are intended toinclude plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises” or“comprising,” when used in this specification, specify the presence ofstated features, integers, steps, operations, elements components and/orgroups or combinations thereof, but do not preclude the presence oraddition of one or more other features, integers, steps, operations,elements, components and/or groups or combinations thereof.

As used herein, the term “and/or” includes any and all possiblecombinations or one or more of the associated listed items, as well asthe lack of combinations when interpreted in the alternative (“or”).

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this invention belongs. It will befurther understood that terms, such as those defined in commonly useddictionaries, should be interpreted as having a meaning that isconsistent with their meaning in the context of the specification andclaims and should not be interpreted in an idealized or overly formalsense unless expressly so defined herein. Well-known functions orconstructions may not be described in detail for brevity and/or clarity.

It will be understood that when an element is referred to as being “on,”“attached” to, “connected” to, “coupled” with, “contacting,” etc.,another element, it can be directly on, attached to, connected to,coupled with and/or contacting the other element or intervening elementscan also be present. In contrast, when an element is referred to asbeing, for example, “directly on,” “directly attached” to, “directlyconnected” to, “directly coupled” with or “directly contacting” anotherelement, there are no intervening elements present. It will also beappreciated by those of skill in the art that references to a structureor feature that is disposed “adjacent” another feature can have portionsthat overlap or underlie the adjacent feature.

Spatially relative terms, such as “under,” “below,” “lower,” “over,”“upper” and the like, may be used herein for ease of description todescribe an element's or feature's relationship to another element(s) orfeature(s) as illustrated in the figures. It will be understood that thespatially relative terms are intended to encompass differentorientations of the device in use or operation in addition to theorientation depicted in the figures. For example, if the device in thefigures is inverted, elements described as “under” or “beneath” otherelements or features would then be oriented “over” the other elements orfeatures. Thus the exemplary term “under” can encompass both anorientation of over and under. The device may otherwise be oriented(rotated 90 degrees or at other orientations) and the spatially relativedescriptors used herein interpreted accordingly. Similarly, the terms“upwardly,” “downwardly,” “vertical,” “horizontal” and the like are usedherein for the purpose of explanation only, unless specificallyindicated otherwise.

It will be understood that, although the terms first, second, etc., maybe used herein to describe various elements, components, regions, layersand/or sections, these elements, components, regions, layers and/orsections should not be limited by these terms. Rather, these terms areonly used to distinguish one element, component, region, layer and/orsection, from another element, component, region, layer and/or section.Thus, a first element, component, region, layer or section discussedherein could be termed a second element, component, region, layer orsection without departing from the teachings of the present invention.The sequence of operations (or steps) is not limited to the orderpresented in the claims or figures unless specifically indicatedotherwise.

1. General Methods and Apparatus.

Suitable additive manufacturing apparatus include those configured forcarrying out bottom-up additive manufacturing. Such methods are knownand described in, for example, U.S. Pat. No. 5,236,637 to Hull, U.S.Pat. Nos. 5,391,072 and 5,529,473 to Lawton, U.S. Pat. No. 7,438,846 toJohn, U.S. Pat. No. 7,892,474 to Shkolnik, U.S. Pat. No. 8,110,135 toEl-Siblani, U.S. Patent Application Publication No. 2013/0292862 toJoyce, and US Patent Application Publication No. 2013/0295212 to Chen etal. The disclosures of these patents and applications are incorporatedby reference herein in their entirety.

In some embodiments, the additive manufacturing step is carried out byone of the family of methods sometimes referred to as continuous liquidinterface production (CLIP). CLIP is known and described in, forexample, U.S. Pat. Nos. 9,211,678; 9,205,601; 9,216,546; and others; inJ. Tumbleston et al., Continuous liquid interface production of 3DObjects, Science 347, 1349-1352 (2015); and in R. Janusziewcz et al.,Layerless fabrication with continuous liquid interface production, Proc.Natl. Acad. Sci. USA 113, 11703-11708 (Oct. 18, 2016). Other examples ofmethods and apparatus for carrying out particular embodiments of CLIP,or of additive manufacturing, include but are not limited to thosedescribed in B. Feller, US Patent App. Pub. No. US 2018/0243976(published Aug. 30, 2018); M. Panzer and J. Tumbleston, US Patent AppPub. No. US 2018/0126630 (published May 10, 2018); K. Willis and B.Adzima, US Patent App Pub. No. US 2018/0290374 (Oct. 11, 2018);Batchelder et al., US Patent App pub. No. US 2017/0129169 (May 11,2017); Sun and Lichkus, US Patent App Pub. No. US 2016/0288376 (Oct. 6,2016); Willis et al., US Patent App Pub. No. US 2015/0360419 (Dec. 17,2015); Lin et al., US Patent App Pub. No. US 2015/0331402 (Nov. 19,2015); and D. Castanon, US Patent App Pub. No. US 2017/0129167 (May 11,2017), the disclosures of which are incorporated by reference herein intheir entirety.

2. Implementation of Resin Viscosity Sensing by Platform ForceDetection.

FIGS. 2-3 schematically illustrate an apparatus useful for carrying outthe present invention. In general, the apparatus includes a light engine10, a window (or “build plate”) 12, a controller 13, and elevator anddrive assembly 14. A carrier platform (or “carrier plate”) 15 is mountedto the elevator and drive assembly as in conventional apparatus, butwith a force sensor 16 operatively associated therewith. The window maybe provided as a “cassette” having a frame 11, which cassette isremovable from the overall apparatus. A polymerizable liquid or resin 21is provided on top of the window 12, the resin having a fill level B-B,which fill level is preferably positioned between a maximum fill levelA-A and a minimum fill level C-C. The three sets of arrows facing oneanother in FIG. 3 illustrate the impact of the carrier platform with theresin surface portion when the carrier platform is advanced to theresin.

The window 12 may be impermeable or semipermeable to an inhibitor ofpolymerization (e.g. oxygen), depending on which particular approach forcarrying out additive manufacturing is employed. In some embodiments,the window comprises a fluoropolymer, in accordance with knowntechniques.

Any suitable light engine 11, including any of a variety of lightsources and/or patterning elements, may be used, including lasers (e.g.,scanning lasers as in traditional stereolithography), liquid crystaldisplay (LCD) panels, digital micromirror displays (DMDs), etc. A singlelight engine may be used, or a tiled set of light engines may be used,depending on the size of the window 12 and the desired resolution.

While the schematic suggests that advancing is accomplished by loweringthe carrier on the elevator, note also that advancing may be achieved byproviding a fixed or static carrier, and by mounting the window andlight engine on an elevator beneath the same, which can then be raised.

Any suitable device may be used as force sensor 16. Examples include,but are not limited to, mechanical tactile sensors, capacitive forcesensors, metal strain gauges, semiconductor strain gauges, conductiveelastomers, carbon felt and carbon fiber sensors, piezoelectric forcesensors, pyroelectric force sensors, optical force sensors, magneticforce sensors, ultrasonic force sensors, electrochemical force sensors,etc., including combinations thereof. See, e.g., Matthias Fassler, ForceSensing Technologies (Swiss Federal Institute of Technology Zurich,Spring Term 2010). One suitable example is the Omega LCM202-1KNMiniature Metric Universal Load Cell, available from Omega Engineering,Inc. (800 Connecticut Ave., Suite 5N01, Norwalk, Conn. 06854 USA). Anysuitable configuration of the force sensor or load cell may be used,including but not limited to a single load cell mounted (or“sandwiched”) in-line between the carrier and the elevator. The forcesensor may include multiple force sensors providing an averaged output(e.g., sandwiched between a compression plate to equalize load), and/ormay include multiple force sensors providing independent data frommultiple regions of the carrier. In addition, force sensing can becarried out by sensing motor current or torque, or any other direct orindirect measure of force.

Note that, where interchangeable cassettes are not employed, or whereall cassettes have the same dimensions and volume, the steps ofconverting resin level to resin volume described in the methods andsystems above can be eliminated.

In some embodiments, the force sensor may be also used by the apparatusto determine when resin flow into the build region is substantiallycompleted, to facilitate or speed production of objects with theapparatus, such as described in R. Truong, Continuous Liquid InterfaceProduction with Force Monitoring and Feedback, PCT Application WO2018/111533 (published 21 Jun. 2018).

Generating the viscosity measurement can be carried out in any suitableprocessor, including the apparatus controller 13, a cloud-basedprocessor, etc. The viscosity measurement can be generated from theposition and force data by known techniques with an empirical model,such as a regression model, based on actual data from a correspondingstandard resin or sampling of resins. The model can be updated asadditional resins are sampled, as in a machine learning method, again inaccordance with known techniques.

Aspects of the present invention are explained further in the followingnon-limiting example.

Example

During the initial descent of the platform towards the cassette windowin one example of a bottom-up stereolithography apparatus (e.g., aCarbon Inc. M1 apparatus, available from Carbon Inc., 1089 Mills Way,Redwood City, Calif. USA), the platform load cell experiences forces dueto resistance from the resin (FIG. 1). After reaching a pre-specifieddistance from the window, the platform remains at this distance and thecompressive forces on the platform relax, as shown on the right side ofFIG. 1. The velocity of the platform is not constant during thisdescent, as the platform slows as the distance from the windowdecreases. Hence, the shape of the force profile does not follow asimple asymptotic relationship. Additionally, there is variabilitybetween print jobs in the platform descent velocity, and the finaldistance between the platform and the window, which influence the shapeof the force profile. Thus, it is a challenge to identify features ofthe force profile that are correlated with resin viscosity, whileremaining insensitive to parameters of the print job.

As shown in FIG. 3, using platform force and position time series data,various features are extracted which are strongly correlated with resinviscosity. Examples of such features are the force on platform atspecified distances with the window, or the half-life of the compressiveforces while the platform remains at a fixed distance. Other examples offeatures consider combinations of printer signals, such as the forcedivided by the platform velocity at a specific location, or thederivative of the force with respect to the platform height.

A statistical model is then developed to predict resin viscosity giventhese features. In order to build this model, a dataset for whichviscosities and the aforementioned features of time series data areknown must exist. Using this dataset, a model that maps these featuresof the force and platform position data to resin viscosity is developed.It is also preferred to generate a dataset for which a wide range ofviscosities are well defined. For this purpose, a viscosity standard,silicone oil, was used. A spectrum of viscosities was generated bycontrolling the temperature of the oil. The oils viscosity at a fixedtemperature was measured using a viscometer; the force profile of aprint job was obtained at a corresponding temperature, and from thesedata a model was generated.

Two such models were created and found to give good predictions ofviscosity: multi-variate linear regression, and random forestregression. These models are widely used in the field of machinelearning, but are quite different from one another. Linear regression isa parametric method which assumes that the values of features linearlyscale with the viscosity, and that features are independent from oneanother. Random forest regression is a more flexible model, whichconsists of an ensemble of decision trees. The value of the viscosity ispredicted as an average of many decision trees. Random forest is not aparametric method, and makes no assumptions on the functional form ofrelations between features and the target viscosity. Other models existwhich were also tested for prediction of viscosity, such as supportvector regression.

In order to improve the practicality of viscosity prediction, and thegeneralizability of the model towards predicting viscosity of manyresins, it is desirable to use as few features in prediction aspossible. The best set of features for predicting resin viscosity wasidentified using a forward selection method, during which features wereiteratively added to the model and selected based on minimizing the meanabsolute error of viscosity predictions. The final model makes use a setseveral features to predict viscosity to +/−440 cP (95% confidenceinterval of model error). FIG. 5 demonstrates the predicted and actualviscosities for silicone oil in the printer. While similar levels ofaccuracy are expected to be attainable for stereolithography resins, itwill be appreciated that looser measurement tolerances will beacceptable in situations where slower production speeds, less accurateobject production, or the like are acceptable.

The foregoing is illustrative of the present invention, and is not to beconstrued as limiting thereof. The invention is defined by the followingclaims, with equivalents of the claims to be included therein.

We claim:
 1. An apparatus useful for making a three-dimensional objectfrom a polymerizable resin, comprising: (a) a build platform on which athree-dimensional object can be made; (b) a light transmissive windowhaving a build surface operatively associated with said build platform,said build platform and said build surface defining a build regiontherebetween, said window configured to support a resin pool thereon;(c) an elevator assembly operatively associated with said build platformand/or said window, said elevator assembly configured for advancing saidbuild platform and said window member away from one another to draw saidpolymerizable liquid into said build region; (d) a light engineoperatively associated with said window and positioned to irradiate saidbuild region with light to form a growing three-dimensional object fromsaid resin; (e) a force sensor operatively associated with said platformand/or said window and configured to detect force exerted on said buildplatform upon contacting said resin; and (f) a controller operativelyassociated with said carrier platform, said light engine, and said forcesensor, said controller configured to generate a viscosity measurementfor said resin from said detected force.
 2. The apparatus of claim 1,wherein said window is stationary in the lateral (X, Y) dimensions. 3.The apparatus of claim 1, wherein said light engine comprises a lightsource in combination with a patterning array.
 4. The apparatus of claim1, wherein said force sensor comprises a strain gauge.
 5. The apparatusof claim 1, wherein said controller generates a viscosity measurementwith an empirical model.
 6. The apparatus of claim 1, wherein saidcontroller generates a viscosity measurement with a regression model. 7.The apparatus of claim 1, wherein said controller generates a viscositymeasurement with a machine learned model.
 8. The apparatus of claim 1,wherein said window is permeable to oxygen.
 9. The apparatus of claim 1,wherein said controller is configured to (a) record said viscositymeasure in association with a resin batch identity; and (b) produce anobject on said additive manufacturing apparatus, wherein said producingis modified based on said viscosity measure.